Understanding nighttime methane emissions

Methane in the Amazon

Methane emissions are smaller than those of CO2, and methane abundance in the atmosphere is much smaller. But methane is much more efficient in trapping heat than CO2 is, making it a very important greenhouse gas. Like CO2, methane is emitted by the burning of fossil fuels but it also has many natural sources. They include thawing permafrost and wetlands.

This is where the Amazon rainforest comes into play. The Amazon and its tributaries are bordered by wetlands of continually flooded forest. In addition, river levels swell significantly during the wet season and seasonally flood large areas of otherwise dry forest. Biomass begins to rot in oxygen-depleted water in these wetlands and produces methane. Scientists estimate that up to one-third of all global wetland methane emissions stem from the Amazon rainforest.

Because methane is such an important greenhouse gas, is it crucial that we better understand the natural processes that contribute to methane emissions. We want to figure out what source and sink regions are and how climate change affects them. Yet emissions from tropical wetlands are the single largest source of uncertainty to the global methane budget. Now Santiago Botía and his co-authors analyzed methane in the atmosphere at ATTO. Over the course of five years, they measured methane along with other properties, such as wind speed, wind direction and the stratification of the atmosphere.

Curious methane emission in the dry season

They noticed frequent pulses of methane emissions during the night, but only under certain conditions. The winds always came from the Southeast, the direction where the Uatumã River lies. The atmosphere was also very well stratified above the canopy during those nights and wind speeds comparably low. Surprisingly, these nighttime events mostly occurred in the months of July to September – the dry season in the Amazon. These findings posed a puzzle to our researchers for quite some time.

Methane (CH4) emissions increase during the dry season. Figure from Botía et al. (2020).

After thoroughly analyzing all the available data and rejecting other potential sources like fires and methane emitted in the main Amazon River branch, they came to the conclusion that the most likely source is the Uatumã River. Lots of flooded forest trees and dead stands are located in the Uatuma towards the southeast of ATTO. The Balbina dam built in the 1980s changed the natural flooding pulse along the river, causing massive mortality of flooded forest trees downstream of the dam, reaching all the way to the ATTO site. During the dry season when water levels are low, methane produced in the water column and river sediment might escape more easily from the water and enter the air above. Under just the right atmospheric conditions that occur more frequently during the dry season, winds bring this the methane-enriched air to the ATTO site.

Drowned trees in the Uatuma river. © Martin Kunz / MPI-BGC.

These results are an important first step in figuring out the methane emission dynamics that we observe at ATTO. But lots of questions remain. Going forward, we plan on performing complimentary methane measurements at the river and with drones. In addition, we want to look at isotope ratios, which will help us to decode the methane sources.

Botía et al published the current study Open Access in Atmospheric Chemistry and Physics, Issue 20: Understanding nighttime methane signals at the Amazon Tall Tower Observatory (ATTO).